The proper design and function of many devices requires rigid bonding of diverse materials. In many cases a brittle material, such as quartz or a ceramic is bonded to a metal. Adhesives can be employed for this purpose but do not allow for detachment of the two materials in the event that adjustment or maintenance is required. The use of metal clamps is unsatisfactory because of the danger of breakage of the brittle material. In addition, most metals have a very different coefficient of thermal expansion than quartz or ceramics. Thus, a clamp that is tight at one temperature can be loose at a higher temperature.
Accelerometers are devices that measure acceleration in many applications. Gravimeters (or gravity meters) are extremely sensitive and precise accelerometers that measure variations of the earth's gravitational field. Modern versions of such gravimeters can achieve relative accuracies of the order of a few micro Gals (10−8 m/s2), i.e. a few parts in 10−9 of g, the earth's mean gravitational attraction.
A full review of the design of gravimeters, both historical and current, is found in the volume Gravimetry, authored by Wolfgang Torge, Walter de Gruyter Press, Berlin-New York, 1989. Numerous designs of gravimeters have been proposed and built over the past 100 years or more. Most of these have been based on deflection, by changes in gravity, of a proof-mass that is supported by an elastic spring member. The elastic spring member can take the form of a helical spring (e.g. LaCoste-Romberg, Worden and Scintrex, as described in the Torge reference, pages 232-236) or a torsion wire (e.g. Mott-Smith, Norgaard, and Askania—Torge pages 227-228). Both metal and quartz have been employed for the material of the elastic spring in these various gravimeters. Each material has merits and shortcomings with respect to ease of manufacture and stability with time, with changes in temperature and with shock.
On the whole, quartz appears to be the preferable material for the elastic spring, due to the inherent material properties. Quartz is highly elastic and shows little mechanical hysteresis after extension or torsion. In thin fibres for springs or hinges (for highly sensitive sensors), quartz has very high strength. This permits the use of quartz fibres for springs or hinges in unclamped mode in rough field use, with no deleterious effects. This is shown in, for example, “The potential application of the Scintrex CG-3m gravimeter for monitoring volcanic activity: results of field trials on Mt. Etna, Sicily”, by G. Budella and D. Carbone, Journal of Volcanology and Geothermal Research, 76 (1997)199-214. Because of its' elasticity, quartz is resistant to irreversible offsets caused by sudden shock, known as “sets”. On the other hand, thin metal fibres are very prone to such sets. Also, quartz has negligible magnetic susceptibility, and thus is unaffected by strong magnetic fields, unlike ferrous metals. Quartz is also a good insulator and facilitates the electrical isolation of metallic components that is necessary in the design and proper functioning of some gravimeters. From a manufacturing standpoint, a quartz-based gravity sensor is, in some respects, easier to construct, as complex forms and attachments of other quartz components may be achieved by heat forming.
A problem arises, however, when a quartz-metal joint is required, for example to support the quartz structure, or to attach a metal component to it. It is important to the proper functioning of the gravimeter that such attachments be rigid and stable, allowing no relative movement of the quartz-metal members, while avoiding stress on the quartz during clamping, causing the quartz to shatter. Glue or mechanical clamps are two approaches commonly used to solve this problem
Mechanical clamps are complex and relatively large, which makes them unsuitable for miniature components. Also it is difficult-to-distribute the required clamping force over sufficient contact surface area to prevent damage to the quartz component. This problem was clearly stated in the article “Tidal to Seismic Frequency Investigations with a Quartz Accelerometer of New Geometry”, by Barry Block and Robert D. Moore, Journal of Geophysical Research, 75, No.8, Mar. 10, 1970. To achieve mechanical support of the quartz torsion fibre, Block and Moore ground the quartz to provide flat surfaces for clamping to metal components without slippage. It was determined that the surfaces had to be ground fiat to within 12 microns to mate precisely with the corresponding metal surface. To reduce the possibility of breakage of the quartz, a layer of soft aluminum foil cushioned each clamp. Insertion of the soft aluminum foil reduced the rigidity and stability of the resulting joint.
A second means of creating such joints is the use of an epoxy cement or other type of adhesive. This approach has a number of disadvantages, however. At the microscopic level it does not form a stable and totally elastic bond. It is non-reversible, and does not allow for adjustment, alignment or later maintenance or repair. Epoxies also exude vapours, which contaminate the atmosphere in the gravity sensor and may adversely affect the performance of the gravimeter.
An additional problem that arises through the use of either metal clamps or cements to effect a bond is that there is a large difference in the coefficients of thermal expansion between most metals and quartz or ceramics. A joint created at one ambient temperature may become loose at a higher temperature.
There is a need for a means for rigid and stable attachment of quartz to metal in miniature quartz-element accelerometers such as gravimeters, which does not have the problems associated with mechanical clamps or glue. It is desirable that the attachment be reversible to allow detachment for the purpose of assembly, adjustment or maintenance.
A technique of attachment of a metal part to another metal part by thermal means, commonly known as “shrink-fit”, is well known in the art (e.g. see Timoshenko, S. Strength of Materials 3rd edition 1956-68 Van Nostrand. P36, 205). In practice, this technique is usually carried out using only metal parts. The present invention exploits the difference in the coefficients of thermal expansion between the two materials being joined. In joining quartz or ceramics to metal, the two materials have very different coefficients of thermal expansion. It is this difference that presents difficulties in effecting joints through other means, such as clamping, or through the use of adhesives. Joints created in accordance with this invention are very simple in design, so are suitable for miniaturization.